Experiments have been conducted in the end cells of a tandem mirror to determine the effect of ion cyclotron resonance heating (ICRH) on the ion pitch angle and velocity distribution. An ICRH power of 2-4 W·cm−3 was measured to have both a directed and a diffusive velocity space effect on the mirror trapped plasma ions, which have densities of (4-6) × 1012 cm3 and average perpendicular energies of 600-1000 eV. On low energy streaming ions (10-50 eV, v∥ > v⊥), the ICRH has a directed pitch angle effect which causes a dominant number of the eventually trapped ions to reflect or to mirror near the resonant magnetic field and to generate a sloshing ion like velocity distribution. However, a sloshing ion like axial density profile was not measured. On hotter thermal ions (600-1000 eV) the ICRH has a measurable diffusive effect, which increases with RF electric field and spreads the distribution function 15-25° around the pitch angle of the RF resonance. On hot test particles generated by 6.9 keV neutral beams injected nearly perpendicularly to the axis, Coulomb and charge exchange collisions were found to dominate over RF diffusion. These observations are discussed in the context of RF kick-per-pass and quasi-linear theory.